Basic electrostatics (geometry help)

1. Sep 23, 2011

geft

Here's the problem: http://i.imgur.com/dS7ZH.jpg

I don't understand why the cosine has to be there. My geometry is a bit rusty, but wouldn't it be enough without the cosine part?

2. Sep 24, 2011

Staff: Mentor

E is the vector sum of the 4 E's. The vertical component of each Eo at the point of interest is cancelled out by the vertical component there due to an identical charge at one other vertex, leaving only horizontal components to be added. The cos gives you that horizontal component of each E.

3. Sep 24, 2011

geft

Ah, you're right. Thanks!